Balanced output circuits are commonly used in communications and audio equipment to facilitate the connection of audio frequency signals between separated devices or equipment to reduce or eliminate unwanted effects of hum, interference, and noise introduced into the system by conduction through grounding or radiated fields. In the normal process of connecting different system components in an audio frequency system, potential differences in ground voltages at the location of the different system components often occur. These unwanted hum and noise signals present in the power grounding circuits will be added into the desired signal unless some means are provided to reject the common mode ground noise. Traditionally, the most reliable means of achieving this has been to add output transformers to the coupled signals to break the path through ground and isolate the pure signal from the differences in ground potential. However, transformers have three principal qualities which are undesirable. These qualities include relatively high cost, large physical size and high weight. It is therefore highly desirable to utilize an electronic circuit having low cost, small size and low weight which is capable of performing the function of a transformer in rejecting common mode ground noise.
How common mode ground noise is rejected with a so-called perfect output transformer is as follows in a worst case scenario in which hum is directly coupled onto one of the balanced transmission lines. Considering a piece of audio equipment with a balanced output being supplied over balanced lines to the unbalanced input of a remotely located unit, assuming that the noise signal is introduced within the ground loop, the voltage associated with the noise signal is directly coupled to one of the transmission lines and is reflected back from the remote unit over the grounded transmission line to one end of the secondary winding of the output transformer. Assuming a perfect transformer with infinite isolation to the local common, and zero impedance in the secondary, then this hum voltage will appear on the ungrounded transmission line and there will be exactly equal in-phase voltages on each of the lines. Assuming a differential amplifier at the unbalanced input, with an ideal output transformer, the voltage represented by the hum or unwanted signal is equal in magnitude and equal in-phase at the differential amplifier such that the hum or other extraneous signals are completely cancelled. This is because equal voltages simultaneously appear on its inverting and non-inverting input terminals. The same would be true in a balanced input situation.
If the balanced system can be characterized as equivalent to an input transformer, equal in-phase noise at the primary is not transmitted to the secondary and thus is not reflected in the output. The same can be said for a balanced electronic input stage, with the only difference over the unbalanced case being the amount of noise coupled to the balanced transmission line. Note, with a balanced line, the audio signal to be transmitted results in opposite polarity signals at both inputs to the remote unit.
The ground hum or noise voltage is referred to as the common mode signal and the amount of rejection by which the common mode signal will be attenuated is the common mode rejection ratio, CMR.
In theory, for a perfect isolation transformer, the CMR would be infinite. However, in practice, it is not because of the inability of the transformer to be constructed with a zero resistance secondary and due to the finite impedances that exist in the transmission cable and the input to the differential amplifier.
In the past, there have been several attempts to emulate the characteristics of a perfect output transformer. One such circuit utilizes a simple differential output circuit that provides both non-inverted and inverted output signals of opposite phase. This circuit is typically comprised of two high gain amplifiers, one of which having a non-inverted output and the other of which having an inverted output. While this circuit does provide two out-of-phase signal outputs, and thus can be characterized as providing a balanced output, the output is not floating with respect to common. While the circuit is balanced, it does not reject any ground hum whatsoever. Moreover, it has the obvious short circuit problem when one of the output lines is tied to ground as is the case when such a balanced circuit drives an unbalanced input.
In order to provide a circuit which has some ability to reject hum, in the past, a circuit has been provided with two high gain amplifiers and a network of resistors that enable cross-feedback between the two outputs. This circuit does provide two out-of-phase signal outputs and can therefore be characterized as balanced. It also provides a transformer-like effect and can be considered floating, assuming the resistors are of equal value. Even with equal-valued resistors, what this circuit fails to take into account is that it requires output resistors in the output circuits of each of the differential amplifiers. This means that the output voltages downstream of the output resistors are of different magnitudes. The reason is that the resistor associated with the output of one of the differential amplifiers is loaded by a resistor string that feeds the other differential amplifier along with the load impedance. Thus, even with theoretically perfect resistors, the abovementioned circuit has only about 30db of CMR. While the abovementioned circuit does provide somewhat of a floating output, it is clearly not capable of achieving very high CMRs even when built from perfect components.
Additionally, this circuit has several stability problems associated with the cross-coupled circuit due to the fact that a positive feedback gain of almost one exists around the loop. As a result, unwanted DC offsets exist, and latch up can occur. Also, in the unloaded condition, the amplitudes from the two balanced outputs are never equal. Moreover, the system sometimes oscillates and has poor clipping behavior. It will be appreciated that there is no way of solving these problems in the above circuit because the circuit depends on positive feedback to produce the transformer floating effect. There is therefore a necessity for an electronic circuit that achieves CMRs of 100db and does not rely on positive feedback, with the circuit being both small in size and low in cost.
SUMMARY OF THE INVENTION
In order to provide an improved electronic transformer for transmitting audio signals over a balanced line from one location to an unbalanced input at a remote location, a circuit is provided requiring two high gain amplifiers and several differential summing amplifiers, with circuit operation being entirely independent of any resistor matching. The circuit is theoretically capable of achieving an infinite CMR by utilizing two feedback loops to provide the true effect of a perfect transformer. A first feedback loop forces the differential voltage produced at the output of the circuit to be equal to the input voltage. This is provided by a closed loop feedback system which samples the differential voltage at the balanced output of the circuit and compares this to the input voltage. An error voltage is then produced which, when coupled to a high gain amplifier, is self-adjusting to force the output voltage differential to match the input voltage.
While partially emulating a transformer, this, in itself, is not sufficient to define the exact voltages at each individual output. Only the differential output voltage will be controlled by this first feedback system. For a balanced output circuit to be truly capable of floating, the current from each output must be equal and out-of-phase. This is accomplished by a second negative feedback circuit. The equal, out-of-phase output current condition is a necessary condition since it is the provision of identical noise voltages on both output lines which enables noise cancellation.
In this second feedback circuit, since the output currents flowing out of the circuit cannot be interrupted, resistor shunts ar used through which the output currents are sampled. The voltages produced across these shunts are proportional to the current themselves, both in magnitude and phase. If these two voltages are out-of-phase and equal, then adding them together produces a zero result. If they are not equal and out-of-phase, then their addition will produce a non-zero resultant error voltage. This error voltage, when amplified by a high gain amplifier, is used to produce the second negative feedback system condition. If the sum of the output currents is not zero, then this feedback system corrects the two output voltages until this condition is satisfied by summing this error signal into the two outputs.
This dual feedback system architecture results in a first feedback loop which controls the differential output voltage by driving the two balanced outputs in opposite directions until the differential result matches the input signal amplitude. The second feedback loop drives both output voltages in the same direction until the output currents are equal in magnitude and out-of-phase. The result is that hum or extraneous signals result in exactly equal and in-phase voltages being applied to the input terminals of a remote device. This results either in their cancellation or in the circuitry at the remote location effectively being unresponsive to the hum such that no hum is present at the output.
Note, with these two feedback systems, one driving the outputs in opposite directions, the other driving the outputs in the same direction, the system can correctly represent any combinations of differential and common mode voltages without conflict.
Moreover, since the first feedback system is holding the output differential voltage constant, the differential output impedance is exceedingly low. This exceedingly low output impedance is that which is associated with a perfect output transformer. Also, with exactly equal voltages appearing at both outputs of the circuit, with the external signal being applied to one output, the other output is raised to the exact same voltage as well. The effective result is that the line-to-ground common mode impedance looking into the output of the circuit is extremely high. Again, this is characteristic of the so-called perfect isolation transformer.
All of the above occurs on an instantaneous real-time basis. The benefit of the above circuit is that the quality of the pseudo-transformer performance is strictly controlled by the amount of loop gain used in the two feedback systems. The more gain, the better the performance. Thus, there is no critical resistor matching that limits CMR. Also, two very stable negative feedback loops are employed rather than a overall single unstable positive feedback loop.
What has therefore been provided is electronic transformer which is a high performance balanced output device capable of driving balanced lines with true equivalent transformer balancing action while providing lower distortion and wider bandwidth than actual output transformers. The circuit provides very high common mode rejection ratios approaching and/or exceeding those of actual output transformers and maintains rejection even during clipping overloads. The rejection characteristics of the circuit can exceed, in many cases, those of actual output transformers due to the extremely low line-to-line dynamic output impedance. While practical transformers have a finite resistance between the output signal pair of between 10 and 50 ohms due to the secondary winding, the subject circuit has a balanced output impedance typically less than 0.1 ohms.